The process known as gas chromatography is a powerful analytical tool as well as a valuable separation technique. The process is generally carried out by passing a mixture of gases through a packed column. Once inside the column the gases are sorbed by the packing, retained for a period of time, and then released. The period of time for which the various gases are sorbed on the packing is an inherent function of the interaction between the particular gas and the packing. Since different gases are retained by the packing for different lengths of time, a separation between the various gases is achieved by the time that the gases leave the end of a chromatographic column.
Frequently the column will be packed with only a solid. In this case the gases interact with the solid to achieve separation. This is referred to as gas-solid chromatography. In other instances a solid packing (support) is coated with a liquid (stationary phase) so that the gases interact with the liquid coating. This chromatographic technique is known as gas-liquid chromatography. Many column packing materials are available for use in either or both gas-liquid and gas-solid chromatography. As is readily apparent, the degree of successful separation achieved in any gas chromatographic process is highly dependent upon the type of column packing material.
Various packing materials have been routinely used in gas chromatography. The most prominent materials are polytetrafluoroethylene, diatomaceous earth, silica gel, molecular sieves, and high molecular weight polymers of chlorotrifluoroethylene. Carbon granules, graphitized carbon black, and graphite coated glass beads have also been used as column packing materials. Such packing materials have been used in gas-solid chromatography as well as with coatings in gas-liquid chromatography. Liquid coatings which have been generally used as the stationary phase in gas-liquid chromatography include low molecular weight polymers of chlorotrifluoroethylene and fluorocarbon oils such as the perfluoroalkanes.
The types and combinations of packing materials is thus large. However, even with a large variety of packing materials there are still gases that can't be effectively separated or resolved.